The trend in the design of magnetic hard disk drives is to increase the recording density of a disk drive system. Recording density is a measure of the amount of data that may be stored in a given area of a disk. Current disk drive products use longitudinal magnetic recording technology. Longitudinal magnetic recording is reaching limitations as the areal density is increased. One such limitation is in regards to the width of the recording transitions. Another such limitation is thermal stability of the recorded magnetization transitions. The width of a magnetization transition in longitudinal recording is proportional to the magnetic moment density, MrT, (where Mr is the remanent magnetization measured in units of magnetic moment per unit of volume, e.g., emu/cm3, and T is the film thickness, measured in units of length, e.g., cm) and inversely proportional to the magnetic coercivity, Hc, of the media. Thermal stability of the media is improved by increasing its MrT and its Hc. Large transitions widths limit the storage capacity of the system by limiting the number of magnetization transitions that can be resolved per length of track recorded. Thus, the tendency in the industry has been to increase He and lower MrT to achieve better resolution as areal density increased. However, the maximum value of Hc allowable is bounded by the writing head magnetic field strength and the minimum value of MrT allowable is bounded by thermal stability requirements. One solution to reduce the transition region in the magnetic recording layer of a longitudinal magnetic recording disk is to invoke synthetic antiferromagnetic (SAF) structures. SAF structures dispose a Ruthenium (Ru) interlayer between two hard magnetic recording layers. The Ru interlayer induces anti-ferromagnetic coupling between the hard magnetic recording layers. This anti-ferromagnetic coupling allows for the use of lower effective MrTs while at the same time keeping the transitions thermally stable. This effective reduction of MrT reduces the length of the transition region and improves PW50 (the pulse width where the read head output amplitude, in response to an isolated transition, is 50% of the peak level).
Perpendicular magnetic recording systems have been developed to achieve higher recording density than may be possible with longitudinal magnetic recording systems. FIG. 1A illustrates portions of a conventional perpendicular magnetic recording disk drive system. The disk drive system has a recording head that includes a trailing write pole, a leading return (opposing pole) magnetically coupled to the write pole, and an electrically conductive magnetizing coil surrounding the yoke of the write pole. The bottom of the opposing pole has a surface area greatly exceeding the surface area of the tip of the write pole. To write to the magnetic recording media, the recording head is separated from the magnetic recording media by a distance known as the flying height. The magnetic recording media is rotated past the recording head so that the recording head follows the tracks of the magnetic recording media, with the magnetic recording media first passing under the opposing pole and then passing under the write pole. Current is passed through the coil to create magnetic flux within the write pole. The magnetic flux passes from the write pole, through the disk, and across to the opposing pole. Conventional perpendicular recording disks typically includes a hard magnetic recording layer in which data are recorded, and a soft magnetic underlayer. The soft magnetic layer enables the magnetic flux from the trailing write pole to return to the leading opposing pole of the head with low impedance, as illustrated by the head image of FIG. 1A.
Perpendicular recording disks should have much narrower PW50 than is currently observed in longitudinal recording disks because in a perpendicular recording layer all of the magnetic easy axes are aligned in the perpendicular direction, i.e. the direction of recording. With this perpendicular recording type of media, a soft magnetic underlayer (SUL) is intended to serve as a flux concentrator to provide a sharp head field gradient so that narrow transitions can be written. One problem with current perpendicular magnetic recording disks is that the soft magnetic underlayer, contains magnetic structures that are fully exchange coupled. As such, any magnetization transition present in the soft magnetic underlayer will be at least as broad as a typical domain wall width (e.g., 100 to 500 nm), illustrated in FIG. 1B. Also, the presence of such a domain wall will reduce the local permeability of the SUL and may influence the structure to act as if no soft underlayer material were present in that region to provide a low impedance flux path. A large width of the domain walls acts to degrade the sharpness of the head gradient and limit the value of PW50. This is a problem because sharp head field gradients are needed to write narrow transitions in the perpendicular magnetic recording films. Another problem with domain walls present in the SUL is the field that they project. When the read head passes directly over the media where a domain transition is present in the SUL, it will pick up a corresponding low frequency signal that adds to the noise of the system thus degrading overall performance.
One solution for the transition width problem is to exchange decouple grains in the soft underlayer material. The decoupling can be achieved by adding a material such as silicon dioxide (SiO2) in the soft underlayer material in order to segregate the SiO2 to the grain boundaries and break the ferromagnetic coupling between SUL grains. A reduction in exchange coupling between the soft magnetic grains allows magnetic transitions to exist closer together in the recording medium, resulting in greater data storage density. One problem with only adding a segregate to the soft underlayer material is that it may exhibit less permeability and higher coercivity (Hc) than is typical of soft magnetic materials. In addition, even when inter-granular exchange coupling is broken, the transitions in the soft magnetic underlayer may still be wide due to magnetostatic coupling (the magnetic coupling of individual magnetic dipoles in absence of a magnetic field). This is particularly the case (wide transitions) because of the high moment and thickness and low Hc characteristic of these SUL films.
The extent of the magnetostatic coupling can in turn be effectively reduced in the SUL by invoking appropriate synthetic antiferromagnetic structures which in remanence (i.e., in the absence of an external applied field) minimize the net moment available for long range coupling. With an effectively reduced magnetic moment and, in the presence of low intergranular exchange, the length of the transitions can be made significantly smaller. Furthermore, special design of the structure can be achieved that also reduces considerably the amount of field projected away from the transition, thus minimizing the deleterious influence of the eddy fields from the SUL on the read head.